This invention relates to an improved process for preparing arylcyclobutenes, more commonly referred to as cyclobutarenes.
Cyclobutarenes, and in particular benzocyclobutene, are important intermediates for the preparation of monomeric and polymeric compositions. U.S. Pat. No. 4,540,763 discloses that biscyclobutarenes can be processed to prepare polymeric compositions. These compositions exhibit thermal stability at temperatures exceeding 250.degree. C., chemical resistance to most conventional solvents, good mechanical and electrical properties, and low sensitivity to water. They are useful in advanced composites, adhesives, structural laminates, matrix resins, and planarization resins for the electronics and aerospace industries.
As disclosed in Schiess et al., Tetrahedron Letters, 46, pp 4569-4572 (1978), cyclobutarenes have been prepared by the flash vacuum pyrolysis of an orthomethyl-benzylchloride derivative. For example, the flash vacuum pyrolysis of .alpha.-chloro-ortho-xylene (ACOX) will yield benzocyclobutene. The pyrolysis is performed under vacuum to achieve a low partial pressure of the reactant because the conversion of the reactant to the cyclobutarene prepared increases as the partial pressure of the reactant decreases.
The flash vacuum pyrolysis process has three main problems associated with it. First, expensive refrigeration equipment is required to condense the product and other expenses are required to operate under vacuum. Second, the process forms coke or tar on reactor internals and therefore prevents economical continuous operation. Third, hydrochloric acid, which is produced as a byproduct of the pyrolysis in some instances, is highly corrosive to the vacuum and refrigeration equipment.
Another method of decreasing the partial pressure of the reactant is disclosed in U.S. Pat. No. 4,570,011. This method uses a mixture of the reactant and an inert solvent, such as xylene, to decrease the concentration of the reactant during pyrolysis and therefore decrease its partial pressure. However, this method requires the use of a large quantity of solvent which must be separated from the cyclobutarene and recovered. More significantly, the operating pressure must still be reduced to a preferred pressure between 25 mm and 35 mm of mercury in order to achieve a desirable yield of the cyclobutarene.
In view of the deficiencies of the prior art, a process for preparing cyclobutarenes with acceptable yields at substantially atmospheric pressure is needed. Additionally, a process that sufficiently reduces coke or tar formation on reactor internals to allow continuous operation is needed. Furthermore, a process that aids in separating hydrochloric acid or any other acid produced during the reaction and does not require a large quantity of solvent would be highly desirable.